The present invention relates to devices, especially heating devices, that dispense volatiles from a substrate such as a wick, mat or plug. More particularly, the invention relates to ways to reduce the tendency of such substrates to clog during dispensing.
A variety of devices for dispensing volatilizable materials are well known. Such volatilizable materials may be air scents or deodorizers (e.g. fragrances or masks), pest control materials (e.g., insecticides, insect repellants or insect growth control regulators), allergen control ingredients, disinfectants, or other materials.
In some of these devices a wick (typically an essentially cylindrical wick) is provided which draws volatile from a reservoir to a dispensing area. An upper/outer end of the wick is heated. See e.g. U.S. Pat. No. 4,663,315. The disclosure of this patent and of all other publications referred to herein are incorporated herein by reference as if fully set forth herein.
In other such devices a porous mat is impregnated with a volatile material. These impregnated mats are placed on heating plates to cause the volatile material to vaporize. Examples of this type of mat are provided in U.S. Pat. No. 6,309,986. An example of an electric heater suitable to cause such mats to dispense volatiles is disclosed in U.S. Pat. No. 6,031,967.
In still other types of such devices a porous mat that has been impregnated with a volatile material is held within a device such that a flow of hot gases from a fuel burner (such as a candle) passes by the mat and thereby causes the volatile material to vaporize. An example of this type of device is disclosed in WO 00/78135.
In yet another type of such a dispensing device, moving air is directed against or through a substrate to thereby volatilize material that the substrate has been impregnated with. An example of this type of device is disclosed in U.S. Pat. No. 5,547,616.
Our assignee recently filed a patent application on Jan. 30, 2003 with U.S. Ser. No. 10/354,876. That application covered various improvements in wick, mat and plug structures used to dispense volatile materials. That application disclosed providing an array of granular particles (e.g. sand) coated with a binder (e.g. a thermoset polymeric material) structured to form a network of pores in the wick, with the wick being impregnated with a variety of volatiles. While this device was an improvement for many volatiles, it was not optimal for use with certain others of interest (e.g. those containing pyrethrum, a desirable insecticide).
In this regard, many dispensing devices which rely on heat to dispense actives from wicks, mats or plugs can experience clogging problems for certain volatiles. This can be due to the breakdown of the volatile when heated, or due to reactions or binding that occurs involving the volatile when heated. When such clogging occurs, the release rate of the volatile becomes non-uniform, and may in some cases prematurely stop entirely, thus wasting the remaining active trapped in the substrate. Even where some release continues, the release may be at such a low level that the device is not effective for its designed purpose.
For example, pyrethrum insecticide tends to form non-volatile waxes or polymers upon heating which clog wicks, mats and plugs. Similar problems arise with materials containing terpenes.
One proposed solution to this problem has been to use antioxidants in the impregnation liquid. See e.g. U.S. Pat. No. 4,968,487. However, this can add cost to the formulation, and only has some effect when oxidation is central to the clogging issue. Moreover, in spite of the inclusion of antioxidants, some cross-linking of the isoprene units in certain liquid formulations can occur, ultimately leading to the clogging of the wick, mat or plug anyway. This is particularly a problem for wicks intended for providing long term insecticidal protection.
Thus, there is a need for an improved dispensing substrate that can be used in such dispensing devices with reduced clogging tendency.